Pattern synonym constraints :: Ord a => () => ...

Wed Oct 6 02:25:46 UTC 2021

If you haven't yet, it is probably a good idea to read section 6 of
https://gergo.erdi.hu/papers/patsyns/2016-hs-patsyns-ext.pdf

On Wed, Oct 6, 2021 at 10:23 AM Gergő Érdi <gergo at erdi.hu> wrote:
>
> > I'm afraid none of this is apparent from the User Guide -- and I even contributed some material to the Guide, without ever understanding that. Before this thread, I took it that 'Required' means for building -- as in for smart constructors.
>
> No, that's not what the required/provided distinction means at all!
>
> You should think of both Provided and Required in the context of
> matching, not in the context of building. To be able to use a pattern
> synonym to match on a scrutinee of type T, not only does T have to
> match the scrutinee type of the pattern synonym, but you also must
> satisfy the constraints of the Required constraints -- they are
> "required" to be able to use the pattern synonym. On the flipside,
> once you do use the pattern synonym, on the right-hand side of your
> matched clause you now get to assume the Provided constraints -- in
> other words, those constraints are "provided" to you by the pattern
> synonym.
>
> It is true that the builder could have entirely unrelated constraints
> to either (as in the proposal). The current implementation basically
> assumes that the Provided constraints can be provided because the
> builder put them in.
>
> Does this make it clearer?
>
> On Wed, Oct 6, 2021 at 10:13 AM Anthony Clayden
> <anthony.d.clayden at gmail.com> wrote:
> >
> >
> > Thank you. Yes that proposal seems in 'the same ball park'. As Richard's already noted, a H98 data constructor can't _Provide_ any constraints, because it has no dictionaries wrapped up inside. But I'm not asking it to!
> >
> > The current PatSyn signatures don't distinguish between Required-for-building vs Required-for-matching (i.e. deconstructing/reformatting to the PatSyn). This seems no better than 'stupid theta': I'm not asking for any reformatting to pattern-match, just give me the darn components, they are what they are where they are.
> >
> > I'm afraid none of this is apparent from the User Guide -- and I even contributed some material to the Guide, without ever understanding that. Before this thread, I took it that 'Required' means for building -- as in for smart constructors. So PatSyns aren't really aimed to be for smart constructors? I should take that material out of the User Guide?
> >
> >
> > AntC
> >
> >
> > On Wed, 6 Oct 2021 at 10:53, Richard Eisenberg <lists at richarde.dev> wrote:
> >>
> >> You're right -- my apologies. Here is the accepted proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0042-bidir-constr-sigs.rst
> >>
> >> Richard
> >>
> >> On Oct 5, 2021, at 12:38 PM, David Feuer <david.feuer at gmail.com> wrote:
> >>
> >> To be clear, the proposal to allow different constraints was accepted, but integrating it into the current, incredibly complex, code was well beyond the limited abilities of the one person who made an attempt. Totally severing pattern synonyms from constructor synonyms (giving them separate namespaces) would be a much simpler design.
> >>
> >> On Tue, Oct 5, 2021, 12:33 PM Richard Eisenberg <lists at richarde.dev> wrote:
> >>>
> >>>
> >>>
> >>> On Oct 3, 2021, at 5:38 AM, Anthony Clayden <anthony.d.clayden at gmail.com> wrote:
> >>>
> >>> >    pattern  SmartConstr :: Ord a => () => ...
> >>>
> >>> Seems to mean:
> >>>
> >>> * Required constraint is Ord a  -- fine, for building
> >>>
> >>>
> >>> Yes.
> >>>
> >>> * Provided constraint is Ord a  -- why? for matching/consuming
> >>>
> >>>
> >>> No. Your signature specified that there are no provided constraints: that's your ().
> >>>
> >>>
> >>> I'm using `SmartConstr` with some logic inside it to validate/build a well-behaved data structure. But this is an ordinary H98 datatype, not a GADT.
> >>>
> >>>
> >>> I believe there is no way to have provided constraints in Haskell98. You would need either GADTs or higher-rank types.
> >>>
> >>>
> >>> This feels a lot like one of the things that's wrong with 'stupid theta' datatype contexts.
> >>>
> >>>
> >>> You're onto something here. Required constraints are very much like the stupid theta datatype contexts. But, unlike the stupid thetas, required constraints are sometimes useful: they might be needed in order to, say, call a function in a view pattern.
> >>>
> >>> For example:
> >>>
> >>> checkLT5AndReturn :: (Ord a, Num a) => a -> (Bool, a)
> >>> checkLT5AndReturn x = (x < 5, x)
> >>>
> >>> pattern LessThan5 :: (Ord a, Num a) => a -> a
> >>> pattern LessThan5 x <- ( checkLT5AndReturn -> (True, x) )
> >>>
> >>>
> >>> My view pattern requires (Ord a, Num a), and so I must declare these as required constraints in the pattern synonym type. Because vanilla data constructors never do computation, any required constraints for data constructors are always useless.
> >>>
> >>>
> >>> For definiteness, the use case is a underlying non-GADT constructor for a BST
> >>>
> >>> >      Node :: Tree a -> a -> Tree a -> Tree a
> >>> >
> >>> >    pattern SmartNode :: Ord a => () => Tree a -> a -> Tree a -> Tree a
> >>>
> >>> with the usual semantics that the left Tree holds elements less than this node. Note it's the same `a` with the same `Ord a` 'all the way down' the Tree.
> >>>
> >>>
> >>> Does SmartNode need Ord a to match? Or just to produce a node? It seems that Ord a is used only for production, not for matching. This suggests that you want a separate smartNode function (not a pattern synonym) and to have no constraints on the pattern synonym, which can be unidirectional (that is, work only as a pattern, not as an expression).
> >>>
> >>> It has been mooted to allow pattern synonyms to have two types: one when used as a pattern and a different one when used as an expression. That might work for you here: you want SmartNode to have no constraints as a pattern, but an Ord a constraint as an expression. At the time, the design with two types was considered too complicated and abandoned.
> >>>
> >>> Does this help?
> >>>
> >>> Richard
> >>> _______________________________________________
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> >>
> >>
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